Abstract6LoWPAN networks are 802.15.4-based wireless sensor networks that seamlessly integrate with IPv6 networks using specially designed protocols. Unfortunately, 6LoWPAN networks are vulnerable to hidden wormholes. To detect and subsequently avoid hidden wormholes, Jain et al. recently proposed two schemes, which exploit the reciprocity of received signal strength indicators (RSSIs). However, threats and attacks can mislead Jain et al.'s schemes into reaching false positives or false negatives. Moreover, their schemes take calibrated RSSIs for granted, which is impractical. In this paper, we propose "Secure Channel REciprocity-based WormholE Detection (SCREWED)", which avoids both false positives and false negatives to a great extend. For this, SCREWED uses channel hopping, randomized transmission powers, message integrity codes, as well as a special replay protection mechanism. Furthermore, SCREWED obviates the need for calibrating RSSIs by using a different channel recipro city metric. We integrated SCREWED into the link layer of Contiki's 6LoWPAN stack and demonstrate SCREWED's efficacy using TelosB motes.